All heat engine cycles are inherently limited in maximum theoretical efficiency of conversion of the heat energy content of the external heat energy source supplied, to output shaft power delivered, by the maximum external thermal temperature gradient across which the engine cycle operates. That becomes the temperature range between the peak temperature of the external energy source input to the engine cycle, and the minimum external ambient temperature available to which its exhaust stream may be discharged. The greater the difference in temperature between the external heat energy source and the external ambient temperature, the higher the efficiency.
This maximum potential thermodynamic efficiency of all heat engine cycles is known as the "Carnot cycle" efficiency. The Carnot cycle is a hypothetical thermodynamic cycle containing zero internal sources of energy losses, requiring only infinitely small approach temperature differences for heat energy transfer to occur. The Carnot Cycle efficiency is governed by the equation: ##EQU1## wherein H.S.(.degree. K.) is the temperature of the heat source and C.S. (.degree. K.) is the temperature of the cooling source.
Ambient temperatures vary across both a daily and seasonal range. In most areas of the north temperate zone, and at higher altitudes not mitigated by abutting large bodies of water, daily temperature swings of more than 30.degree. F. (16.70.degree. C.) are common, and below- freezing temperatures are seasonally common from late fall through early spring.
Current practice in power plant installations devoted to generation of electric power for distribution, supplied from an external heat energy source at an elevated temperature produced by burning a fuel of one sort or another, overwhelmingly employ steam as the thermodynamic medium circulating in closed Rankine cycle turbine systems. Efforts to improve efficiency have therefore been concentrated on means of developing the maximum peak temperature of the external energy source supplying the turbine cycle. For the site of a given installation, it has been customary to select the coldest reliable naturally available ambient heat sink to serve the system, and adapt the remainder of the cycle to make best use of whatever portion of that naturally occurring ambient sink temperature as could be effectively used by the steam cycle, and as would remain reliably available year round. However, anything colder than the saturation temperature of steam at a minimal saturation pressure of 1.5" hg.abs. offers little further thermodynamic cycle efficiency improvement potential. The use of 1.0" hg.abs. vacuum conditions to circumvent this problem only compound in-leakage problems and add only a small fraction of the winter time opportunity presented.
Another way to circumvent this inherent limitation of steam as a thermodynamic medium circulating in Rankine cycle engines is through the use of organic fluid media in Rankine engine "bottoming cycles" known as "organic Rankine cycles" (ORCs) to permit development of colder available ambient temperature sinks. Such cycles are used in "combined cycle turbine systems" in which steam is also employed to take advantage of the higher temperatures available from external heat energy sources in common use, and the exhaust temperature reached, after the steam portion of the combined cycle thermal range has been traversed, is transferred to the organic fluid medium for continued expansion down to the coolest ambient sink temperature reliably available year round. U.S. Pat. No. 3,257,806 (the "Stahl patent") discloses an example of a system which employs such a combined organic cycle system.
By choosing from among a range of organic hydrocarbon fluids available, appropriate selections for their use as turbine media, for specific thermal regimens anticipated in an application, permits optimizing their selection for a combination of most useful temperatures and pressures for a proposed cycle at its intended site, including use of whatever lowest available ambient temperature sink might exist there to serve the attainable exhaust discharge pressure as saturation pressure at that coldest available ambient temperature. Media, bracketing the thermal range associated with desired temperature and pressure cycle parameters, may be selected not only for their characteristic pressure/temperature curve relationships, but for the shape of their saturation curves across that range to be advantageously chosen to facilitate selection of cycle paths with minimum entropy values.
In U.S. Pat. No. 5,555,731 (the "Rosenblatt patent"), the content of which is expressly incorporated herein in its entirety by reference, the use of an elevated temperature injection cycle is disclosed as part of a combined power turbine system employing an absorption refrigeration sub-system. Such an injection cycle is used for introducing selected mass flow quantities of turbine medium, at a selected temperature, pressure, and quality, into whatever vapor phase condition in the turbine medium exists at the point of injection chosen. In that process, the injected mass flow, pressure, temperature, and quality may all be selected by the cycle designer. The interaction of that additional mass flow, mixing with the vapor medium in transit, may be chosen to alter temperature, pressure, unit volume, and mass flow along the cycle path beyond the point of injection. In addition, the isentropic path along which the ensuing cycle proceeds from the point of injection, is altered.
The original objective of the Rosenblatt patent was directed toward employing that path control property using injectors so as to minimize the presence of superheat waste heat contributions remaining in the isentropic path as saturation pressure developed at a selected pre- determined condenser temperature value. The Rosenblatt patent however failed to give any consideration to the use of the control property to accommodate seasonal changes in temperature. Specifically, the Rosenblatt patent did not take into consideration changes in the external ambient coolant fluid temperature and how by monitoring such a temperature and subsequently altering the temperature, pressure, unit volume, and mass flow along the cycle path beyond the point of injection, access to the entire annually available external ambient thermal range is maximized in the thermodynamic cycle of a Rankine cycle turbine system is made available.
It is therefore an object of the present invention to provide a heat engine cycle for use in a power turbine engine system which is capable of adapting to changes in external ambient temperature.
It is a further object of the present invention to provide a thermodynamic cycle of a Rankine cycle turbine system which is capable of maximizing access to the entire annually available external ambient thermal range.
It is also a further object of the present invention to provide a bottoming cycle in which the exhaust saturation pressure and temperature conditions of the exhaust are adjusted to match the coldest ambient cooling temperature concurrently available, as it occurs.
It is also an object of the present invention to provide an improvement over the power turbine engine system described in U.S. Pat. No. 5,555,731, whereby the system can be adjusted to accommodate changes in external ambient temperature and in which vacuum conditions in the turbine cycle are eliminated.